1. Field of the Invention
This invention relates to a composition and process for forming a phosphate conversion coating on active metal surfaces in order to increase the corrosion resistance of the surfaces, either as treated or after subsequent conventional overcoating of the conversion coating layer formed by an organic based protective coating such as a paint or lacquer. Unlike many of the other compositions known for this general purpose, a composition according to this invention is well adapted to treating any of a variety of base metals, including at least steel and galvanized steel, zinc and zinc based alloys, aluminum and aluminum based alloys, and magnesium and magnesium based alloys. The composition and method of the invention are therefore especially well adapted to treating objects having surfaces including more than one type of active metal to be protected against corrosion.
2. Statement of Related Art
A wide variety of phosphate conversion coating compositions and processes are already described in the art. Those believed to be most closely related to the present invention are described below.
U.S. Pat. No. 4,865,653 of Sep. 12, 1989 to Kramer teaches the use of hydroxylamine or agents that react in water to produce hydroxylamine in zinc phosphating solutions to expand the range of zinc concentrations over which the most desirable coating morphology for a zinc phosphate conversion coating can be obtained. Ferrous, zinciferous, and aluminum surfaces can all be coated with the compositions and processes taught by this reference.
U.S. Pat. No. 4,637,838 of Jan. 20, 1987 to Rausch et al. teaches zinc phosphating solutions optionally containing nitrobenzene sulfonate, nitrilotriacetate, fluoride and complex fluoride anions, and/or chelators such as citrate and tartrate.
U.S. Pat. No. 4,149,909 of Apr. 17, 1979 to Hamilton teaches using a combination of accelerators including an oxidizing agent such as a chlorate or bromate in conjunction with a reducing agent such as hydroxylamine sulfate to phosphate ferrous metal surfaces at low temperatures to produce an iron phosphate coating with good salt spray corrosion resistance.
U.S. Pat. No. 4,148,670 of Apr. 10, 1979 to Kelly teaches treating aluminum with an aqueous composition comprising a zirconium or titanium compound which may be the fluozirconate or fluotitanate, a fluoride compound which may also be the noted complex fluoride compounds, and phosphate ions.
U.S. Pat. No. 3,619,300 of Nov. 9, 1971 to Heller et al. teaches zinc phosphate conversion coating compositions containing zinc, phosphate, nitrate, and nitrite ions along with a combination of fluoride and bifluorides of sodium and potassium and teaches that such compositions are useful for treating aluminum, iron, and/or zinc based surfaces.
A commercial product of Henkel Corporation sold for phosphate conversion coating processes more than one year before this application contains phosphate ions, nitrobenzene sulfonate ions, hydroxylammonium sulfate, sodium xylene sulfonate, the monobutyl ether of diethylene glycol {i.e., HO--(CH.sub.2).sub.2 --O--(CH.sub.2).sub.2 --O--(CH.sub.2).sub.3 CH.sub.3 }, and surfactant. Another commercial product of Henkel Corporation sold for more than one year before this application for phosphate conversion coating contains phosphate, hydroxylammonium sulfate, sodium molybdate, sodium sulfate, surfactants, an antifoam agent, and a siliceous desiccant. However, neither of these two commercial products contains any simple or complex fluorides or any organic compounds containing two or more hydroxide and/or carboxyl functional groups.